A standard motor-vehicle door latch is mounted on a door edge and comprises a fork or keeper engageable around a bolt projecting from a door post against which the door edge engages when the door is closed. A pawl can normally hold the fork in a position locked around the bolt to maintain the door closed, and can be pivoted by means of an operating lever away from the fork to release the bolt and allow the door to open. This operating lever can be actuated by an inside door handle, an outside door handle, and in a power-lock system by an electrical actuator.
To lock the door, the operating lever is decoupled from the pawl or prevented from moving. This locking action is in turn controlled by mechanism including a locking lever itself movable between an unlocked position in which the operating lever can act on the pawl and a locked position in which the operating lever is either decoupled or blocked, as described. All standard lockable door latches have such a locking lever movable in this manner.
In older vehicles the doors are all provided with inside locking elements, e.g., buttons or levers, that can be manually operated from inside the vehicle to move the locking lever between its positions. The front doors are normally provided with externally operable key cylinders that allow them to be locked and unlocked from outside also. Thus the car can be locked up manually simply by locking the back doors from inside and the front doors from outside. These inside locking buttons and levers make it relatively easy to break into a car, for instance by slipping a wire through the window seal and hooking it on the button or lever. Thus in recent times these inside locking elements have been eliminated for security's sake.
In a standard central power-lock system the locking lever of each door is moved between its positions by its own actuator. All the actuators are connected to a central controller that is itself operated often by a remote controller carried by the operator of the vehicle. Such a system is extremely convenient in that it allows the vehicle operator to lock and unlock all the doors at one time, normally simply by pushing a button on the driver's door or on the remote.
The central-locking system has, however, the considerable disadvantage that, if the system fails to operate, it becomes on the one hand necessary to lock the front doors manually by means of their key cylinders, and impossible to lock the rear doors which as described above nowadays do not have the inside unlocking buttons or levers.
Accordingly German patent 4,108,561 of Theodor Menke proposes a system where a small rotary knob is set in the door edge where it is visible and accessible when the door is open, but hidden when the door is closed. This knob has a central hole adapted to fit a screwdriver or even a car key. It is connected to the locking lever so that it can be pivoted to throw the latch into the locked position. A door equipped with this latch can be locked manually by opening the door to gain access to the unlocking knob mounted on the door edge, then inserting a key or the like in its slot to pivot it into the locked position. Subsequent closing the door blocks ready access to this unlocking knob.
While such a system does allow one to lock a vehicle door when the central-lock system is not operating, it has several major disadvantages. The door-edge locking knob constitutes one more part in the respective door latch, increasing the cost of this mass-production item. Furthermore the latch becomes more difficult to install with one more element that must be lined up with a hole in the door and then tested. Finally the hole in or around this rotary manual-lock element is an opening through which a tool inserted past the door seal can enter the latch and, in the hands of a skilled thief, unlock the door.